I found some flour in my freezer that I stored 5 years ago in Food Saver vacuum sealed bags. Do you think it's still safe to use?
I'm reasonably sure that flour stored this way is safe. Obviously, if there are any obvious issues with the flour - off colors, tastes, or textures, or evidence of poor storage that allowed the flour to thaw and refreeze, or leaked moisture into it enough for quality concerns - you would want to be safe and discard it, but it should be reasonable to expect the flour to still be safe if it was properly stored.
Flour has a shelf life of 5-10 years if well stored. On top of that, freezing usually doubles the shelf life of grains - and that doubled time is best-before time, the flour may still reasonably expected to be safe after that amount of time, just with reduced quality or nutrition. Storing foods in the freezer, if kept consistently at temperature and well wrapped against moisture, is often about loss of quality rather than safety in any case.
So if the storage was uncompromised, flour stored in the freezer should be best used before 10-20 years. As for how long after that your flour may be edible for, well, hardtack has been known to last fifty or a hundred years, and still work for rations without killing or starving the eaters - I would guess probably modern freezers intended for food preservation are at least as efficient as baking the flour into dry dehydrated cakes (which is really all that hardtack is). That may explain why flour is said to last indefinitely if properly stored in a freezer.